Conjugated polymers consist at least of one backbone chain of alternating double and single bonds. The π-electrons in conjugated polymers are delocalized and can be easier moved from one bond to the other, what makes such conjugated polymers to be a kind of organic semiconductors, thus referred as semiconducting polymers as well.
Generally, semiconducting polymers are created via cross-coupling condensation reactions with symmetrical monomer units that produce linear polymer backbone configurations. When symmetric monomer units are involved, various regiochemistries between adjacent structural units along the backbone vector can be created. As an example. Scheme-1 demonstrates the possible polymer chain configurations can be generated from monomer 3-hexylthiophene. The electronic properties such as charge mobility in organic thin film transistor (OTFT) and power conversion efficiency (PCE) in organic photovolatic (OPV) devices have been shown to be strongly dependent on the configuration of polymer backbone. Higher performance has been generally observed for these polymers with higher regioregularity. A high regioregularity means a high degree of heat-to-tail (HT) coupling and a low amount of head-to-head (HH) coupling or tail-to-tail coupling (TT) coupling as illustrated in Scheme-1.

It is still a technical challenge to synthesize a conjugated polymer with a controllable regio-regularity from asymmetric monomer units, and in most cases, the yielded polymer is often a random mixture of variety configurations including HT-HT, HT-HH, TT-HT and TT-HH as illustrated in Scheme-1, therefore the resulted polymer is referred as regio-random. Techniques to increase population of HT configuration, i.e. to increase regio-regularity of the resulted polymers, have been increasingly explored such as these disclosed in WO2005/014691, WO2006/107740, WO2007/146074, WO2008/092490 and WO2009/056490 for examples.
Furthermore, to achieve the desired energy-gap, HOMO/LUMO levels, absorption spectra, charge mobility and film morphology of conjugated polymers, the “donor-acceptor”(D-A) motif has been now widely adapted in designing conjugated polymers; where the donor (D) is an electron-rich monomer unit and the acceptor (A) is an electron-deficient monomer unit. A D-A polymer is a co-polymer that consists of alternating donor (D) units and acceptor (A) units. The “push-pull” behavior between donor unit and acceptor unit generally lowers the copolymer band gap and improves the double-bond character between repeating units.
One of most studied acceptors is 2,1,3-benzothiadiazole (BT). The introduction of one electron withdrawing atom, fluorine into the 2,1,3-benzothiadiazole (BT) results in a fluorine-substituted 2,1,3-benzothiadiazole (FBT), which is asymmetric in nature as illustrated in Formula-FBT.

Similar to homo-polymers like P3HT described previously, a D-A copolymer may be resulted in many varieties of polymeric backbone configurations if either or both of the donor unit and the acceptor unit are chemically asymmetric. Scheme-2 presents an example in a case that the acceptor unit is chemically asymmetric such as FBT, where D stands for a donor unit.

Under traditional cross-coupling condensation reactions between symmetric donor (D) units and asymmetric acceptors (A), the yielded co-polymer would be a mixture of many architectures, i.e. a random co-polymer. It is postulated that producing asymmetry in the polymer backbone could have a significant effect and even slight modifications of the chemical architecture of a conjugated polymer, i.e., single atom substitution and alkyl side chain length, can lead significant changes of copolymer's microstructure and ultimately electronic properties (Chen, H.-Y., etal., Adv. Mater. 2010, 22, 371). Indeed recently, Albrecht etal., (JACS, 2012, 134, 14932), Chen etal., (Nat. Photonics, 2009, 3, 649) and Tseng etal., (Nano Lett. 2012, 12, 6353) reported that semiconducting polymers containing asymmetric units exhibited higher performance in opto-electronic applications.
Evidently, it is now demanded to invent asymmetric conjugated “D-A” copolymers with controlled chemical architecture (regio-regularity) in order to achieve the desired energy-gap, HOMO/LUMO levels, absorption spectra, charge mobility and film morphology, ultimately to yield the desired performances for the targeted opto-electronic applications.
Equally importantly, it is desired to develop the synthetic pathway to reproduce these invented asymmetric conjugated “D-A” copolymers constantly.